The invention relates to mass spectrometry. More particularly, the invention relates to a mass spectrometer having increased ionizing efficiency, sensitivity and resolution.
Mass spectrometers are important analytical tools for determining structural information about molecules. Three of the most common types of mass spectrometers are quadrapole mass spectrometers, sector-field mass spectrometers, and time of flight mass spectrometers.
Time-of-flight (TOF) mass spectrometers operate on the principle that ions having different mass-to-charge ratios (m/z), when projected into a region free of an electric field, will separate according to these ratios. Lighter particles have a shorter TOF over a particular flight distance. If ions travel a fixed distance, d, from an ion source to an ion detector, the total TOF can be determined by the expression:
t=d(m/2zeV)xc2xdxe2x80x83xe2x80x83(1)
where z is the ion charge, e is the electric charge, and V is the accelerating voltage across the path from the ion source to the ion detector. In the case of constant ion energy (the usual assumption), the flight time t, is proportional to the square root of the ionic mass. Consistently with the previous statement, the light ions reach the detector before the heavy ones. Thus, by measuring the flight time, t, from the ion source to the ion detector, the ionic mass can be determined.
TOF mass spectrometers became commercially available in the early 1950""s as a result of the work done by Wiley and McClaren as described in, Review of Scientific Instruments, Vol. 26, 1150 (1955). This reference describes a TOF mass spectrometer with an electron impact ion source using xe2x80x9cspace focusingxe2x80x9d and xe2x80x9ctime-lagxe2x80x9d techniques. After becoming commercially available, TOF mass spectrometers were used for a wide range of applications because of their fast scan capability. The scan capability refers to the amount of time the instrument takes to collect information on all of the different ions present within a selected mass range. However, early versions of these instruments had low sensitivity and low resolution as compared to the other types of mass spectrometers. Recently, much work has been performed to increase both the resolution and sensitivity of TOF mass spectrometers, generally by incorporating laser ionization. For example, U.S. Pat. No. 5,614,711 issued to Li et al describes a TOF mass spectrometer including an ion beam modulator to create ion xe2x80x9cpacketsxe2x80x9d which travel along a flight tube to a detector. The beam modulator, an ion source and the detector are disposed on a common axis in one embodiment. U.S. Pat. No. 6,020,586 issued to Dresch et al describes a TOF mass spectrometer having an ion pulsing region, and a controllable, multipole ion guide. The ion guide is adapted to selectively trap and release ions for detection. U.S. Pat. No. 6,013,913 issued to Hanson discloses a TOF mass spectrometer having two variable reflectrons collinearly arranged between an ion source and ion detector.
The improvements to TOF mass spectrometers described in the foregoing references are substantial, however, there is still a need for a simple, portable mass spectrometer having high sensitivity and low scan time, particularly for analysis of lighter molecules such as gases which occur in very low concentrations.
The invention is a mass spectrometer which includes an ionizer, a collector slit disposed at a predetermined distance from the ionizer, and an electrostatic field source disposed between the ionizer and the collector slit. The electrostatic field has a selected amplitude which is substantially uniform within a space between the ionizer and the collector slit. The electrostatic field is oriented substantially perpendicular to a line between the ionizer and the collector slit. The mass spectrometer includes a magnetic field source arranged to induce a static magnetic field between the ionizer and the collector slit. The magnetic field source induces a substantially uniform magnetic field within the space. The magnetic field has a direction perpendicular to both the electrostatic field and to the line between the ionizer and the collector slit. The spectrometer includes a detector disposed behind the collector slit. The selected electrostatic field amplitude is chosen so that ions having a particular mass number traverse a substantially cycloidal path between the ionizer and the collector slit. The cycloidal path has a linear component which is substantially equal to the selected distance at a position substantially along the line.
One example embodiment of the invention includes a mass spectrometer which has two first parallel electrodes having a selected first electrode potential impressed on them. The spectrometer includes second parallel electrodes disposed between and substantially perpendicular to the first parallel electrodes. The second electrodes each have impressed on them a potential which is selected to induce a substantially uniform electrostatic field oriented perpendicularly to the second electrodes and parallel to the first electrodes. The second electrodes and the first electrodes are disposed in a substantially uniform static magnetic field having a known amplitude, and oriented substantially perpendicular to the electrostatic field and perpendicular to the first electrodes. A first aperture is disposed in one of the first parallel electrodes substantially at one end thereof and at a level of a lowermost one of the second electrodes. The first aperture has a dimension approximately equal to a spacing between successive ones of the second electrodes. The spectrometer includes an electron emitter disposed substantially over the aperture external to the one of the first electrodes. In one embodiment, the emitter is a heated filament. The spectrometer includes a collector slit disposed on the lowermost one of the second parallel electrodes at a selected distance from the aperture. The spectrometer includes a detector grid located substantially behind the collector slit. The grid has a grid potential impressed thereon. In one embodiment, the grid has an internal amplifier such as a microchannel plate located behind the it. The spectrometer includes a selectable voltage source connected to the second electrodes so that a magnitude of the electrostatic field can be selected. Selecting the magnitude of the electrostatic field results in ions having a predetermined mass number traversing a substantially cycloidal path having a component along the lowermost electrode substantially equal to the selected distance. Such ions can pass freely through the collector slit and be detected by the detector grid. Other mass number ions do not pass through the collector slit.
In one embodiment, the spectrometer includes a periodic potential applied to the first electrodes and to the grid. Ions having selected a mass number are collected by the grid when the grid periodic potential is in phase with the first electrode potential. In this embodiment, ions having mass numbers equal to the selected mass number plus an integer are collected at the grid when the grid potential is delayed from the first electrode potential by a selected time delay.